The email that followed Kamala D. Harris’s announcement that she was running for president was simple: six behind-the-scenes photos from her launch day, signed by her “hubby.”

But the note was more than a fuzzy glimpse into the senator from California’s nascent campaign. Co-written by the 28-year-old digital strategist who helped Beto O’Rourke (Tex.) raise tens of millions of dollars online for his unsuccessful bid for Senate , it served as a powerful fundraising tool, helping Harris bring in $1.5 million in the first 24 hours, the campaign said.

Although the Democratic field for the 2020 election is still taking shape, the race for small-dollar donations has already begun, with about two dozen prospective and declared candidates scrambling to build online operations focused squarely on individual supporters who may give in amounts as low as $5.

Historically, these early low-dollar contributions were viewed largely as a sign of grass-roots support and an indication of potential voter enthusiasm. But that has changed in recent elections as small contributions have increasingly filled the coffers of many candidates — providing the fuel that allows them to be viable contenders.

In 2016, Donald Trump shattered the record for presidential small-dollar fundraising, harnessing the energy from his base and boosting the GOP’s digital operations nationwide. His campaign and affiliated committees continue to raise tens of millions of dollars in increments of $200 or less toward his reelection.

In last year’s midterm elections, it was the Democrats who leveraged anti-Trump sentiment and fundraising platforms such as ActBlue to generate record-breaking sums from small donors.

Being able to raise a lot of money from a lot of small donors is now a test — not only of a candidate’s ability to tap this source of funds, but to go head-to-head against President Trump and his army of grass-roots donors.

“It is the metric, 100 percent. The very simple definition of front-runner status is: Can you or can you not raise substantial money online?” said Tom Nides, former deputy secretary of state under President Barack Obama and longtime Democratic fundraiser.

The emphasis on smaller donations is a shift from previous presidential election cycles, when the focus wasprimarily on wealthy donors who spent heavily early on to improve their favored candidates’ chances. These donors and fundraisers would pool massive amounts of money for campaigns and outside groups.

This time, an influx of smaller amounts from a large pool of donors may go further than donations from individuals who give the maximum of $2,800 each, Nides said.

In interviews with The Washington Post, several prominent Democratic donors said they are holding back their support for now, in part waiting to see which candidates successfully attract broad low-dollar support.

The shift also comes amid intensifying efforts among Democratic candidates to distance themselves from the political donor class and moneyed interests.

“This campaign is going to be a community-led effort, and your voice will ring true in everything we do. No corporate PAC money, no federal lobbyist money, no individual Super PAC,” Sen. Kirsten Gillibrand (N.Y.), a declared candidate, tweeted Sunday.

Shelby Cole, the architect behind O’Rourke’s massive email fundraising haul, was hired into a firm led by a top Harris digital consultant shortly after O’Rourke’s loss. Cole is known for bringing a savvy and personal touch to candidates’ email appeals.

“When you feel like you’re being asked by a real human for help, not some robot, you’re more inspired to support that candidate,” Cole said in a statement to The Post. “It’s always nice to be treated like a real person who is actually making a difference as part of a campaign, not a piggy bank.”

But the need to raise millions of dollars through a flurry of emails, texts, tweets and elsewhere on social media will pose a challenge for campaigns: How do you build an online army of supporters that gives tens of millions of dollars? How do you stand out online, when up to two dozen campaigns are targeting similar donors?

For example, Gillibrand’s campaign recently hired two staffers whose job it is to make personalized appeals to online supporters.

Among the first tasks for the digital strategists, Alexis Magnan-Callaway and Justin Jenkins, will be to engage with voters who are interested in Gillibrand casually and may have given her a few dollars, but have not made a decision about which candidate they will support.

“We know we’re in the primary with a lot of really great candidates, and I think a lot of voters love multiple candidates at once,” said Emmy Bengtson, who oversees the Gillibrand campaign’s digital outreach. “Soon, we will be in a place to persuade folks: ‘Why Kirsten.’ But right now, we’re laying a really long-game groundwork, not just to lock in donors but to recruit core supporters who can help our campaign in the long term.”

A handful of potential 2020 contenders has a broad online following that gives them an edge. But others are starting from scratch.

For example, former Housing and Urban Development secretary Julián Castro is trying to recruit staff who can match the firepower of the more established campaigns.

“We don’t have a war chest to set us apart from the more traditional campaigns. But it doesn’t mean that we can’t compete,” said Jennifer Fiore, Castro’s campaign spokeswoman .

Campaigns other than Harris’s declined to provide their fundraising tallies from the first 24 hours, or did not respond to requests for comment.

Overall, many Democratic activists say, the open field of Democratic hopefuls will help the party elevate the strongest contender, with small-dollar fundraising serving as a test of who can best rally the base.

“The explosion of small donors in the party is terrific for democracy and the progressive movement. The less influence from Park Avenue gatekeepers, the better,” said Raj Goyle, a New York Democratic activist.

In a sign that 2020 contenders are also moving to court wealthier donors, several are holding private meetings and phone calls with stalwart party donors in Los Angeles, New York and Washington, according to multiple major donors and fundraisers.

Among the few major Democratic donors who have publicly announced their support for a primary candidate is Susie Tompkins Buell, San Francisco philanthropist and co-founder of the clothing brand Esprit. In an early February email to friends, Buell said Harris’s “campaign rollout has been so strong and so encouraging that I am compelled to get behind her now.”

When asked whether she viewed Harris’s $1.5 million online donation haul as a sign of the strong rollout that wowed her, Buell responded: “Absolutely. She’s on fire,” using an emoticon of flames.